emacs23 cannot find package.el

Hey there!

Thanks for the great emacs package manager! Just what I was looking for :3 Excited to continue to use it as I rewrite my dotfiles from scratch :)

After installing, I noticed that my version of emacs was complaining about a missing package.el (which I've read is built into emacs24). Attempting to run carton commands resulted in the following:
```
Loading 00debian-vars...
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...
Loading debian-ispell...
Loading /var/cache/dictionaries-common/emacsen-ispell-default.el (source)...
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el (source)...
Cannot open load file: package
```

I suspect that this was because running carton was doing so through the emacs batch mode, which was not loading my personal emacs configuration (~/.emacs nor ~/.emacs.d/init.el)

I resolved this by running adding a --user option to the batch command, which apparently loads my emacs configuration :P

I'll attach a corresponding pull request shortly, though the solution does feel a bit brittle; just a quick fix so that I can continue fixing my emacs config :)

This pull request adds in a user option to the emacs batch mode, which will load the current user's emacs configuration, enabling users of older versions of emacs to provide package.el needed for Carton's functionality.